1. Field of the Invention
This invention relates to a radio frequency choke and more particularly to a method of separating a broadband radio frequency signal from an AC power signal by a radio frequency choke that forms a series DC resistance to the radio frequency signal and thus minimizes current loses in the AC power signal and reduces the series resonant effect of the radio frequency choke.
2. Description of the Prior Art
It is well known in the coaxial communications/distribution art, particularly in the transmission of cable television signals and data transmission to utilize a radio frequency choke (RFC) to separate a signal phase 60 Hz AC power signal from a broadband radio frequency signal. As commonly encountered in cable television transmissions the cable signal may be transmitted at a preselected signal at, for example, between 5 to 400 MHz. A signal phase 60 Hz power signal is also transmitted through the same cable that transmits the ratio frequency signal in order to supply power to the amplifiers associated with the cable transmission line. The amplifiers are positioned at selected locations and are powered by the single phase 60 Hz power signal. In cable television systems, the repeater stations transmit the radio frequency cable signal.
The single phase AC power that is utilized to amplify the radio frequency cable signal is transmitted at a current magnitude generally in the range between about 10 to 12 amps and has a peak current as high as 15 amps. The low power radio frequency cable signal is transmitted generally at low voltage, i.e., in the range of 0.3 volt. Therefore, the AC power system at the distribution points in the cable system must be separated or passed by the low power cable signal which is transmitted between 5-400 MHz. This is necessary in order to prevent the higher voltage AC power supply signal from damaging the transformer that receives the low power cable signal. If the high voltage AC power signal is transmitted through the transformer the transformer which represents a short to ground to AC power will be destroyed. Therefore, it is well known to utilize with main line passive devices, such as splitters, directional couplers, power inserters and the like, selected arrangements of radio frequency chokes to separate the 60 Hz AC power signal from the broadband 5-400 MHz cable transmission signal.
A conventional RFC includes a conductor formed of 18 gauge magnet wire wrapped a preselected number of turns around a ferrite coil form. Connected across one of the intermediate coils adjacent one end of the coil and the lead extending from the respective end coil is a resistor which functions as a shunt and has the effect of reducing the impedance of the RFC. This arrangement of wrapping a conductor around a ferrite coil form with the provision of the shunt resistor forms the equivalent of a high Q capacitor in series with the inductance of the wire or what is knwon as a series LC circuit.
One of the undesirable effects of this arrangement is a sharp loss in signal levels at the resonant frequency of the LC circuit. It has been the practice to utilize the parallel resistor across the coil to shunt the inductor in an effort to reduce the effective resonance of the LC circuit. Also in order to reduce the resonance encountered in the LC circuit attempts have been made to reduce the number of turns in the wire of the coil around the ferrite coil form. However, it has been found that this effort has resulted in providing the LC circuit with a resonant frequency which lies outside the passband of the circuit in which it is installed. Consequently, the total inductance of the RFC is reduced to a degree that it is no longer operable at the 5 MHz range.
Additional efforts to reduce the series resonance in an RFC include a reduction in the gauge of the wire forming the RFC coil. This effort has been found to reduce the resonance as a result of increasing the resistance in the wire; however, the overall effect on the LC circuit is a limitation in the amount of 60 Hz AC power that can be passed through the device. As indicated above, it is preferable that a current in the range of 10 to 12 amps and as high as 15 amps be passed through the device.
Another undesirable consequence encountered in the use of a conventional RFC to separate the AC power signal from the radio frequency transmission signal using the shunt arrangement as above described is a lowering of the impedance to ground. This results in an increase in the insertion loss for the respective band width of the radio frequency transmission signal. In addition, a sharp rate of change in the impedance of a reflected transmission signal from a passive unit results in unsatisfactory matching of the input and output signals through a passive unit. If the input return loss is sharp and exhibits a high degree of resonance then the output signal will not be properly matched with the input signal. The desirable situation is to have a substantially linear change in both the insertion loss and input return loss thereby requiring a dampening of the resonance in the coil of the RFC.
Therefore, there is need to provide in main line passive units utilized in cable television, data transmission, telephone communication and the like distribution lines a radio frequency choke that is effective to separate a high voltage AC power signal from a low voltage broadband radio frequency signal having a range between about 5 to 400 MHz. By dampening the series resonance of the radio frequency choke losses in the radio frequency transmission signal are minimized.